This invention relates to ceramic compositions. More particularly, this invention concerns itself with thermal shock resistant refractory oxide composites such as may be utilized as structural materials in the fabrication of crucibles, furnace linings, missile and rocket components as well as other related structures which encounter high temperatures during their operation.
The development of ceramic materials characterized by an improved resistance to failure by thermal and mechanical fracture has become a problem of particular importance with the recent advances achieved in rocket technology and especially in the development of antenna windows for advanced reentry vehicles. A number of materials have been suggested during the many attempts to solve this problem. For example, various ceramic oxides and nitrides such as aluminum oxide, beryllium oxide, magnesium oxide and boron nitride are potential candidates for use as antenna window material in advanced reentry vehicles because of their combined refractoriness and high temperature dielectric property. However, the oxides are susceptible to failure by thermal fracture at ultra high temperatures while boron nitride is difficult and extremely expensive to fabricate into the large monolithic pieces needed for antenna window construction.
In attempting to provide these ceramic materials with increased resistance to fracture by thermal shock, emphasis has been placed on efforts to inhibit or arrest crack propagation. Recent theoretical evaluations indicate that increased resistance to thermal shock can be attained by inhibiting a propagating crack. Heretofore, the inhibition of crack propagation has been accomplished somewhat successfully by the addition of small amounts of certain metal powders to a ceramic oxide matrix.
With the utilization of this invention, however, it has been found that an even greater resistance to thermal shock can be achieved by the incorporation of boron nitride flakes as an inert, non-reactive phase which is dispersed uniformly throughout a refractory oxide matrix such as alumina, beryllia, magnesia or thoria. The combination of boron nitride and the refractory oxide forms a composite which eliminates the deficiencies of either the oxide or the nitride when considered individually as single phase materials.